The Missouri senator runs attack ads not on her opponent but one of his supporters.

Team McCaskill is already employing the Democratic Party’s go-to tactic this midterm: character assassination. There’s not much else. The economy is humming, the party’s centrist and liberal wings are fighting, and the drumbeat of impending Trump doom isn’t finding much accompaniment. So in Missouri as elsewhere, candidates are reverting to personal attacks. But the McCaskill forces are piling on a guy who isn’t even running.

.. Indeed, they are attacking a private citizen and donor, David Humphreys. Back in March, Chuck Schumer’s Senate Majority PAC began plowing millions into attacks on the businessman, who donated to Mr. Hawley’s campaign for attorney general. The pattern is the same: An ad makes a malicious accusation against Mr. Humphreys, then sidles over to tar Mr. Hawley with guilt by association. Just how invested are they in this strategy? Since airing their first spot, 70% of Democratic ads—amounting to $4.7 million—have been focused on Mr. Humphreys.

.. Ms. McCaskill’s pickle is that the GOP has upped its recruitment game. Her only prior re-election bid in 2012 had her face off against Todd Akin, who self-immolated after his blundering comments on abortion and rape. Mr. Hawley—a savvier, younger man and squeaky clean—hasn’t provided a similar opening. A native Missourian and onetime U.S. Supreme Court law clerk, he arrived on the political scene only in 2016, becoming the Show Me State’s first Republican attorney general in 24 years.

.. Mr. Humphreys is a long and active participant in Missouri civic life. He’s been a major backer of judicial reform, so the trial bar loathes him. He pushed hard for the state’s recently enacted right-to-work law, so unions loathe him. He sits on the boards of the free-market Cato and Acton institutes, so liberals in general loathe him.

.. A liberal organization, Campaign for Accountability, sought to keep the affair in the news by filing an official complaint with a federal prosecutor in Missouri. It may now wish it hadn’t. Mr. Humphreys’s attorney recently got a letter from U.S. Attorney Timothy Garrison, stating that his office had followed protocol and referred the issue to the FBI, which determined that “there was no basis for further inquiry.”

.. That won’t stop the attacks because they serve a greater purpose. Beyond the smears against Messrs. Hawley and Humphreys, such ads are a warning to other donors. Don’t get involved, or your reputations and businesses will be next.

.. Intimidation and threats, leveled against private citizens, are now standard liberal practice.

.. It isn’t about transparency in the public interest; it’s about identifying new political targets.